Graft unsaturated rubbery polymer-styrene interpolymer compositions are widely used for molding, e.g., for injection molding, etc., but for many purposes are unsatisfactory because of low impact resistance and other reasons. The impact resistance of polystyrene and other vinyl aromatic polymers has been enhanced by polymerizing monomeric vinyl aromatic compounds in the presence of a rubbery material. The use of such rubbery polymers has brought about an increase in the impact properties of the vinyl aromatic materials but not to the extent that it is considered desirable for many purposes. The rubbery polymers that have been previously used in conjunction with monomeric styrene or other vinyl aromatic compounds have included natural rubber and the synthetic unsaturated rubbery polymeric derivatives of conjugated 4-6 carbon atom diolefins, as for example, GR-S and butadiene-acrylonitrile rubbery polymers. Most of such interpolymers commercialized as high impact polystyrene in the past have utilized GR-S rubbers which are also known as SBR rubbers.
Stoops et al, U.S. Pat. No. 2,754,282, filed Jan. 15, 1953 disclose polybutadiene-styrene interpolymers wherein the polybutadiene is formed by polymerizing butadiene at a temperature below about 60.degree. C using free radical polymerization. The graph on page 343 of the standard treatise Synthetic Rubber by G. S. Whitby, John Wiley & Sons, Inc., New York, 1954, shows that a polybutadiene so formed at a polymerization temperature of 60.degree. C has a cis content of 23.8 percent and a 1,2-addition content of 19 percent. The graph further shows that as the temperature decreases in the polymerization, the cis content and the 1,2-addition content of the resulting polybutadiene also decrease so that at a temperature of -30.degree. C the cis content is 9 percent and the 1,2-addition content is 16.3 percent.
The lower the temperature in the polybutadiene polymerization, the more suitable is the polybutadiene for making graft polybutadiene-styrene interpolymers, as disclosed by Stoops et al at column 2, lines 41-44. Specifically, Stoops et al state that a polybutadiene produced in the polymerization at 10.degree. C showed outstanding qualities in the polybutadiene-styrene interpolymers, which polybutadiene, according to the above-referred to graph of Whitby has a cis content of 15.6 percent and a 1,2-addition content of 17.5 percent.
Stoops et al also teach that use of a polybutadiene produced by the polymerization at temperatures above 60.degree. C results in inferior polybutadiene-styrene interpolymers. Such a polybutadiene-styrene interpolymer is disclosed in Example 6 of Amos et al, U.S. Pat. No. 2,694,692, filed Aug. 25, 1950. According to the graph on page 343 of the aforementioned Whitby publication, such a polybutadiene has a cis content of 27 percent and a 1,2-addition content of 19 percent.
Other references having a general disclosure of graft unsaturated rubbery polymer-styrene interpolymers include the following U.S. Pat. Nos.: Stein et al, 2,886,553; Baum, 2,957,833; Allen, 3,062,777; Safford et al, 3,151,184; LeFevre, 2,460,300; Popielski, 2,893,976; and Porter, 3,485,894.
The most widely commercialized interpolymer in this field has been the SBR-styrene interpolymer.
This invention relates to improved polymer compositions. In particular, it relates to compositions of vinyl aromatic polymers improved by the incorporation of rubbery polymers as to, in illustration, impact properties. More particularly, it relates to such styrene polymers including styrene interpolymer compositions having high impact strengths.
In accordance with the present invention, we have found that molding compositions of vinyl aromatic materials generally can be substantially improved for example, as regards impact resistance. This invention provides compositions comprising polymers of a monovinyl aromatic monomer, the impact values of which have been substantially enhanced by the incorporation therein of an effective amount of at least one percent by weight based on the total polymer weight of a 1,4-polybutadiene having a cis content of at least 25 percent and a vinyl or 1,2-addition butadiene content of not more than about 10 percent. Desirable polymer compositions ordinarily comprise 80-99 percent of a polymer of a monovinyl aromatic monomer and 20-1 percent total rubbery polymer, preferably 10-1 percent, said rubbery polymer comprising (a) at least 1 percent by weight of the total product of a 1,4-polybutadiene having a cis content of at least 25 percent and a vinyl or 1,2-addition content of not more than 10 percent and (b) up to 19 percent by weight of the total polymer weight of another rubbery polymer. Thus, the novel compositions can contain in some instances other rubbery polymers in place of a portion of the noted 1,4-polybutadiene, such combinations of rubbery polymers providing at times advantageous properties in the compositions.
The compositions of this invention can be prepared as by the novel free-radical polymerization methods provided hereby. Generally speaking, they are formed by free-radical polymerization of the vinyl aromatic monomer in the presence of the rubbery polymers as by employing bulk, solution suspension, or emulsion polymerization techniques.
The compositions resulting from such polymerization are referred to in the art as interpolymers, as may be seen from the aforementioned Amos et al U.S. Pat. No., 2,694,692 and Porter U.S. Pat. No., 3,485,894.
The vinyl aromatic monomers employed in this invention include styrene and its derivatives such as halostyrene, alkyl substituted styrenes, aryl substituted styrenes, vinyl derivatives of naphthalene, and the like. Specific examples of such vinyl aromatic monomers include vinyl toluene, bromo styrenes, phenyl styrenes, vinyl naphthalenes, chlorovinyl naphthalenes, and vinyl phenanthrenes. Alpha-substituted vinyl aromatic compounds can also be utilized, especially in combination with the polymerizable vinyl aromatic compounds having no alpha substituent.
The rubbery polymers which may be employed in this invention in conjunction with the high cis content, low vinyl content 1,4-polybutadiene as hereinabove defined include natural rubber and synthetic unsaturated rubbery polymeric derivatives of conjugated 4-6 carbon atom diolefins, for example, rubbery copolymers such as butadiene and styrene, acrylonitrile and butadiene, isoprene and styrene, acrylonitrile and isoprene and 2,3-dimethyl butadiene and styrene copolymers, polyisoprene, polychloroprene, etc.
1,4-polybutadiene having such high cis content and low vinyl content can be prepared by the use of heterogeneous catalysts, such as a chromium compound absorbed on alumina and silica, aluminum tri-alkyl with a metal halide such as a titanium chloride, titanium iodide, and the like halides, organolithium, etc. The particular method of forming the 1,4-polybutadiene can vary so long as it suitably provides 1,4-polybutadienes having cis content of at least 25 percent and vinyl content of not more than about 10 percent. Such polymerizations are normally carried out in a hydrocarbon medium. Typical high cis content, low vinyl content stereoregular 1,4-polybutadienes utilized in this invention comprise about 25 to 95 percent and beyond to 100 percent cis content, about 0-70 percent trans content and not more than about 10 percent vinyl content.
In addition to the required minimum content of 1 percent high cis 1,4-polybutadienes having low vinyl contents, the remainder of rubbery content can be under this invention another rubbery polymer such as natural rubber, synthetic unsaturated rubbery polymeric derivatives of the conjugated 4-6 carbon atom diolefins or mixtures thereof.
In the following examples, comparative data are shown for varying amounts of vinyl aromatic monomers and rubbery polymers. It will be noted that where an equivalent amount of high cis content, low vinyl content polybutadiene is substituted for conventional rubbery butadiene-styrene copolymer or emulsion polybutadiene, the impact strengths of the resultant products are greatly increased and that where intermediate amounts are employed, an intermediate increase in impact strength results.
other advantages besides enhanced impact values at various temperatures have been surprisingly observed for the impact polymer compositions provided hereby. For example, objects have been molded from the compositions having improved surfaces, e.g., both as to smoothness and gloss. It has been observed that parts can be provided using these impact compositions which show significant enhancement of heat distortion temperatures, creep rupture and yield times and flex modulus values, without sacrifice in impact values. Another observation, as shown by Example 26 hereof, has been that the impact compositions provided by this invention have demonstrated greatly reduced retentivity of water, as in the case where the final polymer compositions are provided in the form of beads or granules by aqueous suspension polymerization. This lower retentivity of water property is important since the retained water must be at a very low level prior to actual molding of useable objects therefrom.
Small particles of the polymer compositions provided hereby can be rendered expandable by incorporation therein of an amount of a liquid blowing agent having a boiling point in the range of about 10.degree. to 100.degree. C. Desirably, about 2 to 10 percent of the blowing agent depending on the polymer, blowing agent, etc., can be employed. The blowing agent can be incorporated by known methods, such as by impregnation of the particles or by polymerization in the presence of the blowing agent, in order to provide expandable particles of the polymer compositions with the blowing agent evenly distributed through the particles. The blowing agent is desirably an aliphatic hydrocarbon such as pentane, cyclopentane, heptane, petroleum ether, and the like. The blowing agent can be selected from other hydrocarbons such as lower halogenated hydrocarbons, for example, methylene chloride, halogen substituted ethanes, etc. The expandable particles can be formed into foam articles by known procedures such as closed molding procedures.
In the following examples, parts are recited as parts by weight of the total polymer composition and viscosities are recited for 8 percent weight solutions of the final compositions in toluene at 25.degree. C.